An onomatopoeical adjective based on the sound emitted when something is "oh so tasty" (either through hunger or flavorological value) that one gnaws through it without regard to cleanliness or etiquette. This sort of ravenous eating will often result in an "om nom nom nom" noise being emitted from the eater.

Arthur: Gosh, I haven't eaten for days.
Dirk: Me neither. We're both starving equally.
Arthur: Look! A whole cooked chicken with side of mashed potatoes and a warm/soft dinner roll!
Dirk: We have to decide which of us gets to eat first. To be fair, we should rock-paper-scissors for it.
Arthur: Om nom nom nom nom!
Dirk: Arthur, you're a jerk. A real knee-biter.

The sound of ravenous eating. Popularized by the character Cookie Monster on Sesame street, it is now used commonly in speech, especially among younger people. Cookie Monster exclaims "OM NOM NOM NOM" when gobbling down cookies (and whatever else goes in his mouth).

You can "om nom" on things besides food... for example, play-eating babies' toes to make them laugh.

omnomnomnom (dot) com is a pop culture website where people submit their own silly photos that have been manipulated in Paint to resemble monsters eating various things, including people and pets.